1. Field of the Invention
The present invention relates to a selectively ion-adsorptive, porous membrane. More particularly, the present invention is concerned with a porous membrane which is suitable for use in the purification or removal of desired ionic substances including not only ionic inorganic substances but also ionic organic substances, such as a protein and an amino acid, by selective adsorption on a commercial scale. The present invention is also concerned with a process for producing the selectively ion-adsorptive, porous membrane.
2. Discussion of Related Art
Generally, an ion exchange resin has been used in a laboratory for selective purification or selective removal of a desired ionic substance, such as a protein and the like, for example by ion exchange chromatography technique in which an ion exchange resin having selective adsorptivity properties is utilized. However, ion exchange chromatography is not advantageous due to its poor efficiency. Further, a large amount of an eluent and a long period of time are required for desorption of an adsorbed substance from the ion exchange resin, leading to a disadvantage in the practice of this method. Therefore, ion exchange chromatography has not been put in practice on a commercial scale.
To solve this problem, it has been suggested that a membrane capable of selectively adsorbing a desired ionic substance be used for purifying the ionic substance. Such a membrane would be advantageous in that the efficiency of its adsorption is excellent and therefore, selective purification and selective removal of a desired substance can be conducted efficiently and the time necessary for purification or removal of a desired substance can be shortened.
In this connection, it is known that in order to selectively purify or remove a desired ionic substance by adsorption, a porous membrane comprising a polymer having a side chain containing an ion exchange group, such as an amino group, a sulfonyl group and a carboxyl group, is suitably used. However, such a porous membrane has a disadvantage in that the membrane adsorbs not only the desired ionic substance but also undesirable substances due to the non-specific adsorptivity of the polymer. Such a non-specific adsorptivity property causes the efficiency in the selective purification or removal to be decreased.
Particularly, for enabling the above-mentioned porous membrane to be used for a commercial-scale purification or removal of a desired substance, the polymer skeleton of the porous membrane must have sufficient mechanical strengths. Therefore, a hydrophobic, porous membrane is preferably used. However, the hydrophobic, porous membrane has a drawback in that non-specific adsorption of a protein and the like is likely to occur, leading to a lowering in the efficiency of purification.
For avoiding the above-mentioned disadvantage, it is preferred that the surface of the hydrophobic membrane be rendered hydrophilic with the aid of a functional group containing a neutral hydroxyl group. The term "neutral hydroxyl group" used herein is intended to define an alcoholic hydroxyl group directly bonded to an aliphatic hydrocarbon. It is not intended to define the hydroxyl group of a carboxylic acid or a hydroxyl group directly bonded to a benzene nucleus, such as a phenolic hydroxyl group. A method for rendering the surface of the hydrophobic membrane hydrophilic is disclosed in, for example, U.S. Pat. No. 4,794,002. In this Patent, it is disclosed that the overall surface of a hydrophobic membrane is coated with a compound having a neutral hydroxyl group and an ion exchange group, or coated with a compound containing a neutral hydroxyl group and having a hydrophilic portion and a hydrophobic portion, followed by the addition of an ion exchange group to the coating layer. However, the addition of the ion exchange group to the surface of the membrane has been conducted physically. Therefore, when the membrane is autoclaved, the added ion exchange group is easily removed from the surface of the membrane. Accordingly, the membrane cannot be used repeatedly. Further, in order to use the membrane repeatedly, it is necessary to desorb the ionic substances which have been adsorbed on the membrane. For performing desorption of the adsorbed ionic substances, the membrane is washed with an alkali solution. However, even when a crosslinking treatment is conducted for the membrane, the membrane still lacks resistance to an alkali and therefore, would not satisfactory for use on a commercial scale. Further, since the coated compound is likely to be cleaved from the membrane by a physical action, the membrane cannot be used repeatedly on a commercial scale.